Leveling high speed plating



Dec; 29, 1970 CQWEN EI'AL 3,551,301

LEVELING HIGH SPEED PLATING Filed April 14, 1966 V -HIGH SPEED CONVENTIONAL 80- Z80 LL. Lz')7o I{316C} U 3 40- (n O I F E 0.25 0.5 0.75 LOO U30 PLATE THCKNESS 7 E20 INVENTORS O drzfiw' (bwen,

l n l x 1 I a 15 30456o BY kfidro 75270 A ANGLE OF GRIT LINE T0 FLOW w ATTORNEY United States Patent 3,551,301 LEVELING HIGH SPEED PLATING Arthur Cowen, Detroit, Lynn T. Davies, Pontiac, and

Richard L. Teno, East Detroit, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Apr. 14, 1966, Ser. No. 542,671 Int. Cl. C23b 5/02 US. Cl. 204-29 5 Claims ABSTRACT OF THE DISCLOSURE Improving the leveling of electroplated, polished workpieces by immersing the workpieces in a stream of flowing electrolyte and orienting the polishing grit lines at an acute angle to the direction of flow of the electrolyte.

This invention relates to high speed electroplating processes and more specifically to a method for improving the leveling characteristics of such processes.

Leveling can be defined as that property or characteristic of a plating solution or process which produces a surface smoother than that of the substrate. It is the usual practice to polish steel sheet before it is shaped into the desired part configuration. This polishing operation leaves extremely fine grit lines on the surface of the steel sheet that remain in the surface of the formed part. In order to get a perfect specular surface, as is needed for decorative chromium plating, these grit lines must be obscured. In the past this has been done by applying a heavy layer of copper or semibright nickel to the surface of the part before the conventional bright nickel layer and chromium overlay are applied. If the copper or semibright nickel smooths out (levels) these grit lines, no bufling is required before the bright nickel layer and chromium overlay are applied. If insufficient leveling is obtained butfing is needed to smooth out the surface.

Some attempts have been made toward the direct adaptation of organic bright nickel baths to improve leveling. Baths of this type are characterized by the addition of brighteners such as coumarin and its derivatives, acetylenic alcohols, and nitrogen containing ring compounds such as pyridines, quinolines, and quaternary compounds such as pyridinium and quinolinium compounds. These techniques have been somewhat effective in conventional tank-type electroplating as well as the high speed electroplating processes of this invention. High speed plating as used herein refers to those electroplating processes where the electrolyte is flowed past the workpiece at rates generally in excess of 5 feet per second so as to permit satisfactory metal deposition at current densities up to and even in excess of 1,000 amperes per square foot (a.s.f.).

We have discovered a process which when used in conjunction with high speed plating processes will substantially reduce, and in most cases eliminate, the requirement for recourse to the prior arts preplating with subsequent buffing process steps.

It is an object of our invention to improve the leveling characteristics of high speed plating processes.

It is a further object of our invention to efiect leveling in high speed plating processes by incorporating therein controlled workpiece orientation steps.

It is a further object of our invention to provide for the leveling of high speed electroplated coatings by a process which produces parallel polishing grit lines on the workpiece and then orients the workpiece in relation to the electrolyte flow so as to effect an angle between the polishing grit lines and the direction of electrolyte flow across the face of the workpiece. These and other objects Patented Dec. 29, 1970 and benefits will become apparent from the following detailed description of but one specific embodiment encompassed within the scope of our invention.

FIG. 1 is a partially sectioned elevational view of a device employing the invention.

FIG. 2 is a partially sectioned plan view of the device of FIG. 1 taken along section line 22 of FIG. 1.

FIG. 3 is a graphic representation of the benefits obtained by our invention.

FIG. 4 graphically compares high speed to conventional plating.

Briefly stated, our invention involves the improvement of the leveling capabilities already inherent in high speed electroplating processes and machines such as are disclosed in copending US. patent application S.N. 212,318, High Speed Plating Method and Apparatus, filed July 25, 1962. We improve these processes by polishing the workpiece, prior to plating, in substantially one direction, so as to leave substantially parallel grit lines on the surface. Subsequently the workpiece is oriented in the electrolyte flow channel so as to have the substantially parallel grit lines oriented at an angle to the direction of electrolyte flow across the face of the workpiece. While generally any acute angle between the orientation of the grit lines and the direction of electrolyte flow results in improved leveling, we have found that the larger acute angles approaching the perpendicular/transverse effect the greatest improvement (see FIG. 3).

Frequently the configuration of a particular part will not permit uniform unidirectional polishing. In these cases, polishing sequences or configurations should be so designed as to insure that the maximum portion of any surface can be polished in such a way as to present the maximum grit line angle possible under the circumstances to the direction of electrolyte flow. Likewise since the important aspects of the invention lay in presentation of the workpiece to the electrolyte with its polishing grit lines at an angle greater than zero relative to the direction of electrolyte flow, it is immaterial what means are utilized to effect this end.

FIG. 1 is illustrative of the employment of our invention in a high speed electroplating device of the type disclosed in the aforementioned S.N. 212,318. The plating device may consist of a tank 1 for holding an electrolyte 2. The electrolyte 2 is circulated to the plating channel 13v by means of pump 4. The electrolyte is pumped via conduit 3, through a screen or uniform flow control means 5 and into plating channel 13. The electrolyte 2 flows between the nonconsumable lead anodes 11 and the workpiece 9. Workpiece 9, having grit lines 10, is passed into and through plating channel 13 by means of a holder 12, which is in turn a part of a channel sealing means or top 8. The top 8, and hence the workpiece 9, is moved from one channel to another in any desired plating sequence by conveyor means extensions 15. The top 8 can have sealing means 16 to prevent loss of electrolyte.

The tank 1 may serve many purposes other than merely holding the electrolyte. It may, for example, be itself an electrolytic regeneration cell wherein the metal ion is replenished by the anodic dissolution of a pure metal electrode. It might also serve as a makeup mixing tank in addition to its function as an electrolyte holding tank. In effect, the only thing of significance here is that fresh nondepleted electrolyte be pumped from the tank 1 to the plating channel 13. How the electrolyte is replenished is not a part of this invention. Electrolytes for these systems are maintained fresh by any of a variety of known techniques, such as anodic dissolution replenishment, metal salt addition, and/or the like. As in substantially all high speed electroplaters, the anodes 11 are of the fixed dimension nonconsumable type.

FIG. 3 shows one graphical representation of the leveling improvements that can be obtained by varying the angle of the grit lines to the electrolyte flow. As is clearly shown in FIG. 3, the most significant improvement was noted where the grit lines were at 73 to the direction of flow. For workpieces plated in accordance with our invention, leveling improvement in the order of 70% to 73% was observed. Percent leveling improvement is herein defined as,

(orig. fin.) (final finish) original finish (RMS) For comparison purposes, similar samples were run concurrently with the test samples, varying therefrom only by having the grit lines parallel to the direction of electrolyte flow. The samples at zero grit line angle exhibited only 28% leveling. This 28% was felt to represent approximately the inherent leveling characteristic of high speed platers using electrolytes employing conventional brighteners. It is apparent therefore that under exactly the same conditions our invention has effected a 40% to 45% leveling improvement over the samples not prepared in accordance with our invention.

We have found that at a constant current density the leveling improvement begins even at fiow rates in the order of 5 feet per second. As the fiow rate is increased, the leveling improvement becomes more discernible.

We have found that the thickness of the plate contributes materially to the degree of improvement obtainable. Thus, FIG. 4 shows a comparison between the leveling improvement obtainable by high speed plat ing verses conventional plating as a function of the plate thickness required to effect a desired finish. We see, for example, that commencing with a 47 RMS finished sample, a 23.4% improvement (36 RMS) is possible With but 0.22 mil plate under high speed plating conditions whereas a like improvement under conventional plating conditions would require a 0.77 mil plate.

We have found as a practical matter that there are no significant advantages obtainable by usage of our process to improve samples having a surface finish rougher than 160 RMS. The time and thickness of plate required do not justify the efforts. Likewise, with starting samples having a roughness in excess of 160 RMS it is difficult to practically obtain final finishes much better than 18-20 RMS. In fact, there is a point above which the incident process will have no appreciable effect whatsoever. We know, for example, that a test sample displaying a roughness of 350 RMS could not be improved even with a 1.0 mil plate Whereas the same sample was improved to 140 RMS with a 0.58 mil conventionally plated deposit. It is apparent, therefore, that somewhere between 160 RMS and 350 RMS there lay a point beyond which leveling improvement is practically impossible using this technique. Therefore, though we recognize that somewhat higher RMS surfaces than disclosed herein may be improved by our process, we find it impractical to do so.

EXAMPLE Polished steel plates having an initial finish of 9-10 RMS were processed in accordance with the above comparison procedure.

100 X percent The tests were conducted in a high speed plater of the type disclosed in the aforementioned copending application S.N. 212,318. A nickel bath containing 11.3 oz./gal. of nickel and 0.85 lb./ gal. of Perfiow was used. The bath was maintained within a pH range of 4.0 to 4.9 and a temperature range of to F. The electrolyte flowed across the surface of the respective samples at a rate of 45 to 50 feet per second. A current density of 1,000 amperes per square foot was applied for a total plating time of 76 seconds. The samples which were oriented at 90 to the direction of electrolyte flow in accordance with our invention displayed a 2-3 RMS final finish, whereas the samples maintained at zero grit line deviation displayed a 6-7 RMS final finish.

Though our invention has been described in terms of certain preferred embodiments, it is to be understood that others may be adapted and that the scope of our invention is not limited except by the appended claims.

We claim:

1. A process for effecting the leveling of electrodeposits in a high speed plating process comprising the steps of polishing the surface of a workpiece in substantially one direction so as to produce polishing grit lines substantially parallel to said one direction on said surface, cleansing said workpiece, immersing said workpiece in an electrolyte, flowing said electrolyte across said surface, orienting said grit lines at an acute angle to the direction of flow of said electrolyte across said surface to improve the leveling of said electrodeposit, and passing current through said electrolyte and said workpiece to effect metal deposition on said workpiece.

2. A process in accordance with claim 1 wherein said angle is from 62 to 90 inclusive.

3. A process in accordance With claim 1 wherein the rate of said electrolyte flow is at least about 5 feet per second.

4. A process in accordance with claim 3 wherein the metal is deposited for a time sufiicient to produce a deposit of at least 0.22 mil thick.

5. A process in accordance with claim 3 wherein metal is deposited for a time sufiicient to produce a deposit of at least 0.22 mil but not more than 1.0 mil thick.

References Cited UNITED STATES PATENTS 1,886,218 11/1932 Olin et a1. 20426X 3,007,854 11/1961 Smith et a1 204207X 3,065,153 11/1962 Hough et a1. 204237X 3,071,521 1/1963 Ehrhart 204237X 3,272,727 9/ 1966 Schmeckenbecker 20429 3,277,553 10/ 1966 Wesolowski 20429X FOREIGN PATENTS 986 1896 Great Britain 204237 124,361 4/ 1945 Australia 20426 JOHN H. MACK, Primary Examiner W, B. VAN SISE, Assistant Examiner 

